The present invention relates to improvements in color cathode ray tubes. More particularly, it relates to a cathode ray tube display having a combined degauss and surge limiting circuit.
A typical color cathode ray tube display includes a degauss coil and associated circuitry which allows demagnetization of the CRT's shadowmask. In known color monitors such as the IBM 3279, 5279 and 5379 displays, a coil is fixed around the CRT and is energized upon power-on by alternating current from mains voltage supply fed through a thermistor having a positive temperature coefficient. The current causes the thermistor to heat up, thus limiting the current in an exponentially decaying fashion. Additional resistors or thermistors are required to maintain the temperature of the current limiting thermistor to reduce the current to a minimum level so as not to cause interference in the CRT.
Typical degauss circuits suffer from the following disadvantages:
(a) A wide range thermistor is required to limit the current. PA0 (b) Power is always dissipated by the heating elements. PA0 (c) The thermistor is relatively costly. PA0 (d) There is a current surge into the coil upon switch-on. PA0 (e) Some time, about ten minutes, after power-off is required for the thermistor to cool sufficiently to enable an effective degauss current to flow upon subsequent switch on. PA0 (f) Normally, different circuits are required for different mains voltages in order to satisfy the conditions of maximum and minimum current limits.
The last two disadvantages have led to the use of a separate external switch to supply the degauss coil but this is clearly an undesirable additional expense.
Many CRT televisions and monitors, for example the aforementioned IBM 5279 display, use a switched mode power supply (SMPS) and some method is required to limit the large current pulse in the mains supply when power is connected to the SMPS to avoid damage thereto. The conventional method of limiting the surge current is to use a low-value high-power resistor (for example 10 ohms, 10 watts) in series with the supply to the primary supply (input) rectifiers of the SMPS. This has the disadvantage of dissipating considerable power during normal operation. The problem is worsened by the need of the SMPS to cope with a large range of input supply voltages. The problem has been overcome in the past by either using a thermistor having a negative temperature coefficient to limit the current, the decreasing resistance of the thermistor with increasing temperature reducing power dissipation when the supply is operating, or by using a triac powered from a winding on the SMPS transformer to short the resistor out when the supply is running. The first method is not practical unless a thermistor with a very fast thermal recovery time is used and the second method is usually regarded as an extravagance given the limited function of the circuit.